1. Field of the Invention
This invention relates to the reduction of NO.sub.x emissions produced in a large-scale glass melting furnace by controlling the furnace exhaust gas temperature.
2. Technical Considerations
Large-scale melting of glass entails the combustion of large amounts of fuel in a melting furnace in order to provide the required melting temperatures by direct heating. The fuel (generally natural gas or fuel oil) is usually mixed with an excess of air beyond that theoretically required for complete combustion in order to assure that complete combustion of the fuel actually occurs within the furnace, and particularly in the case of flat glass melting operations, to assure that oxidizing conditions are maintained within the furnace. This combination of conditions within a glass furnace is conducive to the oxidation of nitrogen in the combustion air to NO.sub.x.
NO.sub.x is a shorthand designation for NO and/or NO.sub.2. In the high temperature conditions of a glass melting furnace, the oxide of nitrogen formed is mostly NO with some small amount of NO.sub.2, but after exhaust gas containing NO is released to the atmosphere, much of the NO is converted to NO.sub.2, which is considered to be an objectionable air pollutant and is believed to be involved in the chemistry of smog formation. Therefore, large volume combustion sources, such as glass melting furnaces, are susceptible to governmental regulations that may severely restrict their operations.
A non-catalytic process for selectively reducing NO to nitrogen and water by injecting ammonia into an exhaust gas stream is disclosed in U.S. Pat. No. 3,900,554 to Lyon. This process may be employed on glass melting furnace exhaust streams as disclosed in U.S. Pat. No. 4,328,020 to Hughes. This patent teaches that effective ammonia reduction of NO.sub.x occurs when the exhaust gas from the furnace is at a temperature in the range of 870 to 1090.degree. C. (700 to 1090.degree. C. when the ammonia is accompanied by hydrogen) and further that such temperature conditions exist or can be created within the flue which connects the primary and secondary regenerator chambers of the furnace's heat recovery system for a substantial portion of each firing cycle in a glass melting furnace. The patent further teaches that ammonia injection is discontinued whenever the temperature of the furnace exhaust gas passing through the flue falls outside this preferred operating range. Although this method is capable of removing a large portion of NO.sub.x from glass furnace exhaust, its overall effectiveness is reduced by the ineffectiveness of the ammonia reduction technique during selected portions of each firing cycle of the melting furnace when the exhaust gas temperatures are unsuitable.
To prolong the effective operating time of an ammonia injection system, U.S. Pat. No. 4,372,770 to Krumwiede et al. discloses a glass melting furnace wherein additional fuel is injected into and burned with excess oxygen in the furnace exhaust gas as it enters the primary regenerators during selected portions of the glass melting process, to increase the furnace exhaust gas temperature so that it is within the desired temperature range for effective NO.sub.x reduction by ammonia injection.
Although these types of systems serve to reduce the NO.sub.x emissions, there are still times during the firing cycle of the glass melting furnace when NO.sub.x emissions are not being reduced by use of the ammonia injection system.
It would be advantageous to have a glass melting furnace arrangement which provides both increased use and more efficient use of the ammonia injection system to further reduce NO.sub.x emissions.